1. Field of the Invention
The present invention relates generally to an antenna array apparatus and a method thereof in a mobile telecommunication system, and in particular, to an antenna array apparatus and a beamforming method using a GPS signal in a base station (BS).
2. Description of the Related Art
The rapid increase in the number of users in the present mobile telecommunication field imposes capacity constraints on mobile telecommunication systems. Mobile telecommunication technology has transitioned from the narrow band second-generation phase to the wide band third-generation phase to increase capacity. At the same time, antenna array signal processing techniques are attracting more attention because they can further increase capacity by time-spatial signal processing.
When a conventional omni-directional antenna is used, a signal from a mobile station (MS) interferes with signals from other mobile stations. However, use of an antenna array apparatus enables selective transmission/reception of a signal to/from a particular direction. Consequently, the interference with other mobile stations is reduced and more users can be accommodated.
Signal processing by the antenna array apparatus, applicable on forward and reverse links, is usually adopted in a base station due to the physical size of the apparatus and calculation requirements in a mobile telecommunication system including base stations and mobile stations. The basic structures of a reverse (receiving) beamformer and a forward (transmitting) beamformer are illustrated respectively in FIGS. 2A and 2B. As shown in FIGS. 2A and 2B, antenna devices in the transmitting beamformer and the receiving beamformer having their respective weights and beam patterns in transmitting/receiving arrays are controlled by adjusting those weights. For convenience sake in mathematical terms, a set of weights is called a weight vector.
Many methods have been proposed to calculate an optimum weight vector for use in forming a receiving beam from an intended direction in the receiving antenna array system.
DOA (Direction of Arrival)-based beamforming techniques include maximum SINR beamforming, ML beamforming, and MMSE beamforming which differ in cost functions for achieving an optimum vector. These techniques are readily interpreted theoretically but have the problem of high calculation requirement for eigen decomposition and multi-dimensional, non-linear optimization of an input correlation matrix necessary to implement the techniques.
Another feasible approach is use of a training signal. An LMS (Least Mean Squares) algorithm and a DMI (Direct Matrix Inversion) algorithm are implemented based on training signals. The training signal methods, though it is not necessary to know the DOA methods and an array structure, may have problems related to preliminary symbol and carrier recovery caused by using additional training signal, decreasing channel efficiency.
Weight vectors are also achieved using the characteristics or structure of a signal, such as a CMA (Constant Modulus Algorithm) and an FA (Finite Alphabet). These methods are not greatly influenced by a variety of propagation situations and require no knowledge of the DOA methods and an array structure. However, they have problems in convergence speed and performance.
Meanwhile, calculation of an optimum weight vector is a challenging issue to the transmitting antenna array system to transmit a signal in an intended direction. As opposed to reverse channel estimation, forward channel estimation is difficult in a base station. There are many suggestions to solve this problem.
In the conceptively simplest way, a mobile station estimates a forward channel in a mobile station and transmits the information on a reverse channel. Then a base station calculates a transmission array weight vector based on the received information. Considering the trend of miniaturization and lightweight in mobile stations, this method imposes too high of a calculation requirement on the mobile station.
As an alternative, weight vectors have been calculated using blind channel estimation without feedback information from a mobile station. The blind channel estimation is difficult to be put to wide use because of problems related to local minimum, convergence speed, and initial acquisition. In this context, a semi-blind method has been suggested by combining the above two methods. The semi-blind method aims at maximization of the forward channel estimation capability of a base station and system capacity using part of feedback information obtained with the least calculation requirement from a mobile station.
A weight vector for a reverse channel can be applied to a forward channel. This simple method has limitations in increasing capacity when the difference between forward link and reverse link carrier frequences is greater than a correlation bandwidth in an FDD (Frequency Division Duplex) structure using different frequencies for the forward and reverse links. The method of applying a reverse weight vector to forward channels can be improved by defining a function capable of compensating for the transmission-reception frequency difference between a weight vector for a reception antenna array and a weight vector for a transmission antenna array and achieving a weight vector for the transmission antenna array. However, considering the time varying characteristic of a spatial-time channel , the appropriate function is difficult to be defined.
Some mobile stations in the mobile communication system are equipped with GPS receivers. They receive their position information from GPS satellites and provide the position information to users only, not to a base station.